Along the aerospace history, there are many incidents that have emphasized the important role that aeroservoelastic coupling plays in the stability of controlled vehicles. The instability and handling qualities degradation is to be avoided by suppressing the structural elastic modes (aeroservoelastic coupling suppression) in the feedback paths of the Control Laws.
The Control Laws are any law which is a function of the measured system dynamics, and which governs the movement of the system control devices or effectors, being the effectors of any device intended to modify the movement or displacement of a system.
Aeroservoelastic coupling suppression is a multidisciplinary technology dealing with the interaction of air vehicle non-stationary aerodynamic forces, the structure dynamics and the flight control system dynamics. Several studies have been conducted assessing strategies and methodologies in the design of active flight control algorithms to favorably modify the aeroelastic dynamics of the system, or to simply decouple the rigid and elastic measured dynamics to minimize the adverse effects on the stability margins and handling qualities.
In the particular case of controlled systems with a very flexible structure, it is a normal practice to apply filtering techniques as, i.e., notch filters for removing the elastic modes from the feedback signals. This known technique is suitable for systems with medium to high elastic modes frequencies, in such a way that the elastic mode frequencies lie outside of the control frequency bandwidth of the augmented system.
In the particular case of the flying boom installed in a tanker aircraft, the flying boom is a flexible slender structure with highly non-linear aerodynamics and elastic characteristics that strongly vary with the flight condition, the operational phase, the telescopic beam length and the fuel flow. The first bending mode frequency of the flying boom, both in free-air and coupled conditions (during refueling operations), lies in the bandwidth of the rigid control frequencies.
Alternative solutions to the notch filter for very flexible systems which meet the design requirements are also known. For example, extended Kalman filters can be applied to attenuate the elastic components in the feedback signals at the resonant frequencies using the theoretic model of the rigid system. Additionally, it is known to use a spatial filtering technique that uses a distributed sensor array to cancel the elastic modes components in the feedback signal, assuming the elastic modes shapes are known beforehand. Nevertheless, the number of sensors used in a spatial filtering technique is, in general, greater than twice the number of elastic modes to be suppressed.
These known alternative solutions, as model-based filtering methods, conventionally lack of robustness and adaptation capabilities against plant uncertainties, cannot cope with fast changes in the structure morphology, and their performance is very sensitive to variations in the exogenous boundary conditions acting on the system. Thus, it is essential to perform an alternative method that minimizes the impact on the sensed rigid dynamics component to fulfill the handling quality level and stability margins requirements; and to achieve a robust online cancellation of the elastic modes in the feedback signals using a non-model-based approach.